Session: Benchmarking & cross-cutting Group 1 (Isotope modelling, COMPARE)
Author: Helena L. Filipsson / firstname.lastname@example.org / Lund University, Sweden
Co-author: Daniel C. McCorkle, Woods Hole Oceanographic Institution, USA;
Andreas Mackensen, AWI Bremerhaven, Germany;
Joan M. Bernhard, Woods Hole Oceanographic Institution, USA;
Lars S. Andersson, SMHI, Sweden;
Lars-Johan Naustvoll, Institute of Marine Research, Norway;
Angela M. Caballero-Alfonso, Lund University, Sweden;
Kjell Nordberg, University of Gothenburg, Sweden;
Didrik S. Danielssen, Institute of Marine Research, Norway;
We documented the annual cycle of the stable carbon isotopic composition of dissolved inorganic carbon (δ13CDIC) and the stable oxygen isotopic composition (δ18Ow) in the water columns of the Skagerrak and Baltic Sea and two fjords on the Swedish west coast. The aim was to quantify the isotopic variability and provide more precise palaeoceanographic reconstructions in shelf seas. This is the first study from the region where both δ18Ow and δ13CDIC were measured concurrently, together with hydrographic and nutrient variables, from the same water samples. The lowest δ13CDIC values (-4.9 ‰) were found in the low-oxygen, brackish Baltic bottom water whereas the highest values (+1.8 ‰) were observed in the surface water of the Skagerrak during late summer. Photosynthesis drove the high δ13CDIC values (between 1.0 and 1.8‰) noted in the surface waters of both the Skagerrak and the Baltic. The δ13CDIC values below the halocline in the Baltic reflect mixing of brackish water and the more saline water from the Skagerrak, and foremost organic matter remineralization processes that release significant amounts of low-δ13C CO2. Similarly, in the stagnant fjord basins, little deep water exchange and the degradation of terrestrial and marine organic matter set the δ13C composition. Deep-water renewal in the fjord basins resulted in rapid increases of the δ13CDIC on the order of 1‰, whereas remineralization processes caused a decrease in δ13CDIC of 0.1 – 0.3 ‰ per month depending on location. The combined effects of water mixing and remineralization processes (estimated using apparent oxygen utilization (AOU) values) yielded the expression: δ13CDIC =0.032*S-0.01*AOU-0.12 for the Baltic – Skagerrak region at water depths below the halocline.
The δ18Ow samples the Skagerrak surface water ranged between -0.41 and 0.31‰ and displayed larger monthly variability than the Baltic surface water, which varied between -7.09 and -6.61‰ (depending on site). The very large fresh water supply to the Baltic Sea results in the low δ18Ow values. The maximum in δ18Ow (0.43‰) was noted in the Skagerrak. A mixing line was established for the region (salinity: 6-35): δ18Ow = 0.253*S-8.59 (N=263).